The most successful films of British director Joe Wright announce themselves with a certain cinematic swish, all period charm and romantic froth, whether it be the muddy skirts and country balls of his Pride and Prejudice, or the upper-crust, if doomed, glamour of Atonement. Meanwhile the 40-year-old's directorial debut for the stage, earlier this year at the Donmar, was Trelawny of the Wells, Arthur Wing Pinero's well-made but light-as-a-feather backstage comedy.

So it would be fair to call his second theatrical outing, A Season in the Congo, something of a departure. It's a fast-and-loose version of Aimé Césaire's 1967 play – translated from French, never before staged in the UK – about the Congolese political leader, Patrice Lumumba, who spearheaded the 1960 move for independence from Belgium, and who was assassinated in 1961. To research the subject – which Wright confesses he knew "very little" about – he visited the Democratic Republic of the Congo with Oxfam, witnessing the on-going, deeply destructive legacy of those troubled political times.

The DRC is still blighted by violence and poverty; Wright speaks of the trip slowly, carefully, as if still trying to assimilate it all: "It was a life-changing experience, really …. We spent a lot of time out in the refugee camps in [Eastern DRC], which were shocking. There are two million people homeless, dispossessed, just in that area. I find man's inhumanity to man extraordinary .... I can't get my head around it."

A Season in the Congo is an epic play, in scale and sweep as well as in the Brechtian sense. Alongside a cast of only eight – furiously playing several roles each – there are five dancers, two Congolese musicians (a chorus of sorts) and larger-than-life-sized puppets. The play should be a riot of energy, music and colour as well as having a politically punchy message. For while he says that visiting the DRC was harrowing, Wright also describes with glee the amazing nightlife in the capital, Kinshasa: "Best clubs I've ever been to in my life. Mind-blowing stuff." High praise indeed from a man who used to create music visuals during the rave and superstar DJ era; no wonder he brought their musicians over for the production.

Wright isn't short on collaborators; in charge of movement is the choreographer Sidi Larbi Cherkaoui, who worked on Anna Karenina. "I hero-worship him," says Wright with a little laugh. "There are scenes where Césaire [writes] something like "Civil war". And you go, right: I've got eight actors, how do I do a nation in civil war? And the answer is, I call up Sidi Larbi."

Other collaborators are a little closer to home. Wright's parents ran the Little Angel Puppet Theatre in Islington, north London; his sister Sarah now works there too. She and his mum, Lyndie, are providing handmade, large-scale puppets for the show, which Wright is grinningly delighted about.

He's known for repeatedly using the same high-profiles stars (most notably Keira Knightley), but Wright's got himself a hot new leading man: Chiwetel Ejiofor to play Lumumba. Returning to the stage for the first time since his Olivier-winning Othello in 2008, Ejiofor said yes straightaway. Wright recalls: "He recognised in the play something that is potentially searingly beautiful and true and needs to be spoken about."

It's a story Wright is passionately committed to telling, but it was first suggested to him by David Lan at the Young Vic. Lan, who is South African, like Wright's parents, kept A Season in the Congo in his to-do drawer for years.

Wright insists that Lumumba's being relatively unknown is part of the appeal – unlike other revolutionary icons, such as Ghandi or Che Guevara, there's less of a poster-boy popular narrative. On the Oxfam trip, Wright – along with Lan and Ejiofor – met Lumumba's widow and children, and many of his contemporaries. What impression did he get of this man? Wright is convinced: "He wasn't corrupt."

"He had an ideal, and stuck to his ideal, and he spoke the truth, and he was murdered for it, basically. The Belgians, when they 'granted' independence, thought that they would allow the Congolese political independence but maintain economic control. Lumumba had other ideas; he thought that Congolese wealth should be for Congolese people."

The country has always been rich in natural resources which, the West has coveted at intervals, rubber, copper, uranium and coltan (a key component in mobile phones), Wright says: "It's spooky how they seem to have everything that capitalism has wanted all along."

Césaire's play was written swiftly after events unfolded: Congolese independence, the democratic election of Lumumba, a bloody, secessionist civil war, betrayal, and murder – when the facts were often still murky. In the years since, there have been many revelations about who was pulling which strings. Most significantly, the Belgian government and CIA were responsible for the assassination of Lumumba. It's recently been alleged that MI6 were also involved. What impact does this hindsight have when staging the play today?

"We're not sticking to the exact text that Césaire wrote, and I've tried to bring into the production what we've learnt," says Wright. They've added extra scenes, using text from The Congo Cables, by Madeleine G Kalb, which details communications between the CIA, the Belgians and the UN.

Although he's been afforded an insight into the ongoing troubles of the DRC, Wright won't be bringing the play up to date. But he hopes it resonates. "The same thing is going on in the Congo now as was then: it's controlled by Western, capitalist interests," he says. "That's what killed Lumumba; that's what keeps 90 per cent of the population in poverty. It's a scam. It's a failed state. There's not specific reference to the present government [in the show], but there's reference to the fact that the system in place – how a country like Congo is abused by First World greed – is certainly there."

If this all sounds a long way from those nice Hollywood costume dramas, not only does Wright promise that while "the end of the play is dark … along the way there's a lot of lightness, and extraordinary, uplifting beauty" – he also reveals his next film project. There was talk of Fifty Shades of Grey, before Sam Taylor-Johnson was named director last week. Anyway, "the schedules didn't work".

It's a live-action version of The Little Mermaid, based on a production he saw "thousands" of times, which his late father staged with puppets. It'll be less saccharine than Disney's, which he claims not to have seen, and closer to Hans Christian Andersen's original.

So Wright will continue to straddle the line between theatre and film. He also goes to the theatre more than the cinema, finding London theatre "the most exciting in the world; it's probably why Hollywood films are populated by British actors now". Being part of it this scene is, he hopes, "making me a better film-maker".

"Film has become a very passive experience, but with theatre there is a contract made with the audience, where they participate. That's why my parents' puppet theatre was such a special place – people used their imaginations. It's a muscle that needs using."